lost_in_translation no I am not sure! I very much doubt that anyone can be sure of what is permitted and what is not, especially QR staff!
The only 'official' document we have specifically states Airport upgrades for Qmiles/Qcredits. Nothing else. You can read the document in Post 8 of this thread.
You mention that reward flights are not affected by this change. As far as I know it is almost impossible to obtain a reward flight at the airport. Taking that fact and post 8 into account then it is not unreasonable to assume that upgrades purchased prior to check-in opening are treated the same as reward flights. BUT this is QR!